4 degrees warming "likely" without CO2 cuts: study

Sun Sep 27, 2009 11:00pm EDT

LONDON (Reuters) - Global temperatures may be 4 degrees Celsius hotter by the mid-2050s if current greenhouse gas emissions trends continue, said a study published on Monday.

The study, by Britain's Met Office Hadley Center, echoed a U.N. report last week which found that climate changes were outpacing worst-case scenarios forecast in 2007 by the U.N.'s Intergovernmental Panel on Climate Change (IPCC).

"Our results are showing similar patterns (to the IPCC) but also show the possibility that more extreme changes can happen," said Debbie Hemming, co-author of the research published at the start of a climate change conference at Oxford University.

Leaders of the main greenhouse gas-emitting countries recognized in July a scientific view that temperatures should not exceed 2 degrees above pre-industrial levels, to avoid more dangerous changes to the world's climate.

The IPCC shared the 2007 Nobel Peace Prize for its fourth assessment report, or AR4. One finding was that global temperatures could rise by 4 degrees by the end of the 2050s. Monday's study confirmed that warming could happen even earlier, by the mid-2050s, and suggested more extreme local effects.

"It's affirming the AR4 results and also confirming that it is likely," Hemming told Reuters, referring to 4 degrees warming, assuming no extra global action to cut emissions in the next decade.

One advance since 2007 was to model the effect of "carbon cycles." For example, if parts of the Amazon rainforest died as a result of drought, that would expose soil which would then release carbon from formerly shaded organic matter.

"That amplifies the amount of carbon dioxide that goes into the atmosphere and therefore the global warming. It's really leading to more certainty," said Hemming.

DRASTIC

Some 190 countries will try to reach an agreement on how to slow global warming at a meeting in Copenhagen in December.

Chinese President Hu Jintao won praise for making a commitment to limit emissions growth by a "notable" amount, at a U.N. climate summit in New York last week. Other leaders made pledges to agree a new climate pact.

Temperature rises are compared with pre-industrial levels. The world warmed 0.7 degrees last century, scientists say.

A global average increase of 4 degrees masked higher regional increases, including more than 15 degrees warmer temperatures in parts of the Arctic, and up to 10 degrees higher in western and southern Africa, Monday's study found.

"It's quite extreme. I don't think it's hit home to people," said Hemming. As sea ice melts, the region will reflect less sunlight, which may help trigger runaway effects.

Such higher Arctic temperatures could also melt permafrost, which until now has trapped the powerful greenhouse gas methane, helping trigger further runaway effects, said Hemming.

"There are potentially quite big negative implications."

The study indicated rainfall may fall this century by a fifth or more in part of Africa, Central America, the Mediterranean, and coastal Australia, "potentially more extreme" than the IPCC's findings in 2007.

"The Mediterranean is a very consistent signal of significant drying in nearly all the model runs," said Hemming. A 20 percent or more fall is "quite a lot in areas like Spain already struggling with rainfall reductions in recent years."

(Editing by Andrew Roche)